Devices for preparing bituminous mixes for road surfaces, using virgin aggregates, liquid bitumen and powdery products into which, if necessary, recycled used bituminous mixes have been incorporated, are known. These devices generally consist of a cylindrical drum of large dimensions, mounted on a platform about its axis and slightly inclined relative to the horizontal plane. The virgin aggregates and the powdery products are introduced into the drum at one of its ends. The recycled mix in granular form is introduced via a recycling ring surrounding the drum in an intermediate zone between its two ends.
A burner penetrates inside the drum at one of its ends and enables hot gases to be circulated inside the drum, these hot gases ensuring drying and heating of the materials circulating inside the drum.
Such a drum carries out both drying and heating of the cold and moist aggregates entering into the drum, heating of the recycled mix and mixing of the new aggregates and recycled mix, in contact with liquid bitumen conveyed into the drum through an injection pipe.
The internal wall of the drum is equipped with vanes of varying shapes depending on the zones of the drum so as to ensure, as a result of rotation of the drum, transportation, stirring and/or raising of the materials circulating inside the drum.
Compared to the prior method where drying and heating of the aggregates were performed inside a rotating drier and mixing with the liquid bitumen inside a separate mixer having a fixed casing, the integration of drying, heating and mixing functions inside the same drum has led to a certain simplification of the procedures and materials involved. However, mixing/drying drums have the drawback that they give rise to the presence, inside the same enclosure, of a flame, of very hot gases and of liquid bitumen. This results in bitumen vapor being entrained by the hot gases circulating inside the drum, which in turn leads to rapid clogging of the bag filters used to extract the dust from the gases leaving the drum and to the release of harmful vapors into the atmosphere. These drawbacks are particularly marked in the case of parallel-flow mixing/drying drums, i.e., mixing/drying drums where the hot gases circulate inside the drum in the same direction as the solid materials.
Counter-flow mixing/drying drums have therefore been proposed, i.e., mixing/drying drums where the hot gases circulate inside the drum in the direction opposite to the direction of circulation of the solid materials, in which it is attempted as far as possible to prevent the hot gases produced by the burner from coming into contact with the material containing bitumen, such as the recycled materials or the aggregates being coated with liquid bitumen. To this end, use is made of a burner which has a very elongated body which penetrates inside the drum at its bottom outlet end, so that the end of the burner from where the flame is produced is located in a position relatively removed from the outlet of the drum. The zone for mixing the solid materials with the liquid bitumen as well as, if necessary, the zone for introducing, heating and mixing the recycled mix are arranged between the zone of the drum where the flame is produced and the outlet of the drum, around the body of the burner.
Such devices enable, to a large extent, the formation of bitumen vapor to be avoided, since the hot gases arising from the burner and circulating in the direction opposite to the virgin aggregates introduced via the inlet end of the drum are directed towards this inlet end, i.e., in the opposite direction to the zone or zones receiving bituminous materials.
However, these zones situated downstream of the flame remain exposed to the radiation of the flame, which may cause heating and vaporization of the bituminous products. Moreover, the extension of the flame towards the outlet of the drum is not perfectly controlled, such that this flame or hot gases may come into contact with the bituminous products, when the mixing zone is located immediately downstream of the flame zone.
To overcome these drawbacks, it has been proposed to arrange inside the cross-section of the drum, around the body of the burner, a metal shield in the form of a disk ensuring physical separation between the flame zone and the mixing zone. The need to provide a shield complicates the structure of the drum and reduces the cross-section through which the solid materials pass. Moreover, the shield is subject to degradation and rapid wear owing to the fact that it is situated in a very hot zone.